1. Field of the Invention
This invention relates to a pneumatic radial tire capable of controlling separation failure at belt end.
2. Description of Related Art
There are known pneumatic radial tires for truck, bus and the like, which are particularly used under a heavy load and recapped plural times after the wearing of a tire tread. In this type of the tire, there has recently been caused a problem that cracks are created at a side end of a belt layer in a widthwise direction thereof in the use over a long time of period and grow inward in the widthwise direction or outward in the radial direction between the belt layers with the lapse of time to cause a separation failure and hence make the use of the tire impossible. Such cracks are considered to be created due to the following fact. That is, an end portion of the tread in the pneumatic radial tire is subjected to a tensional deformation in the circumferential direction every the contacting with a flat road surface due to an influence of crown radius. At the tread end portion subjected to such a tensional deformation are located outer side end portions of plural belt layers in which cords are slantly embedded in these layers in opposite directions with respect to an equatorial plane of the tire, so that the cords crossed with each other at this position are subjected to the above tensional deformation every the arrival at the ground contacting region and repeatedly deform in a direction of crushing a diamond shape. And also, such a deformation is caused when the tread end portion of the tire rides on projections such as stones and the like during the running on bad road to largely deform rubber in this end portion. As the cords are deformed as mentioned above, the width of the belt layer becomes narrow and hence the outer end of the belt layer in the widthwise direction somewhat displaces inward in the widthwise direction repeatedly, while rubber located outward from the outer end of the belt layer in the widthwise direction is left at this position. As a result, tensile strain in the widthwise direction is caused at the boundary between the rubber and the outer end of the belt layer in the widthwise direction. Moreover, the ends of the cords embedded in the belt layer are exposed at the outer end of the belt layer in the widthwise direction and are cut faces not subjected to brass plating or the like for enhancing an adhesion to rubber, so that the ends of the cords are repeatedly pulled apart from the surrounding rubber due to the influence of the above tensile strain and hence slight cracks are created in the rubber located in the vicinity of the cord ends exposed at the outer end of the belt layer in the widthwise direction. On the other hand, the aforementioned displacement becomes maximum at a widthwise outer end of a narrow-width belt layer arranged at an outside of a maximum-width belt layer in the radial direction and containing cords arranged in an oppositely inclination direction to cords of the maximum-width belt layer. Therefore, it is considered that the above slight cracks are first created at the widthwise outer end of the narrow-width belt layer and developed by shearing strain or the like repeatedly produced between the belt layers during the running with the lapse of time to cause separation failure at belt end (hereinafter referred to as belt end separation).
In order to solve this problem, there have hitherto been proposed a method of arranging a low-hardness and thick cushion rubber between the widthwise outer end of the maximum-width belt layer and the widthwise outer end of the narrow-width belt layer for controlling the shearing strain produced between the belt layers to prevent the development of the cracks, and a method of piling a reinforcing layer containing cords extended substantially in the circumferential direction on the belt layer for controlling the increase of the size of the belt layer accompanied with the inflation under inner pressure or the running to prevent the development of the cracks.
However, these methods can develop an effect to a certain level, but have a problem that the occurrence of the belt end separation can not sufficiently be controlled.